Had a massive improvement to the final result yesterday. Mostly procedural changes, but I did use wider tracks too. Not tried drilling out the pads yet (it was 01:30) but hopefully the larger pads will resist being ripped up a bit better.

Only a couple of areas under-exposed a bit this time.

Although now I probably want to redesign this without the Arduino, and just use a bare ATMEGA…

Slightly longer exposure (3:00), and a bit of manual rubbing with a soft sponge and/or finger during both the developing and etching process. Seems to help accelerate the processes where needed, so the slower areas ‘catch up’ with the faster areas.

This is a shield for an Arduino Due, containing SDCard, ESP8266-01 and OV2640 camera.
No photo resist, printed using at ancient monochrome laser printer direct to Maplin film, ironed on to bare copper, etched.
What you are doing is absolutely excellent, creating your own light box is admirable, but photo resist PCB material is relatively expensive and spraying your own coating is nigh on impossible (I tried very hard) and all that exposure time testing.
I have seen some film that you apply under water that looks like it gives really good repeatable photo resist on bare PCB, should I find it again?

That toner-transfer method does look very high-fidelity, I like. I didn’t have access to a laser printer at the start of this project, hence my starting down the UV route. Now that I have acquired a laser printer, I might have to give this a go too!

I too heard bad things about the spray-on stuff, so went straight to the pre-applied. Not heard about any water-transfer method before - sounds interesting!

If your board is small and you want few boards, take a look at OSH Park. They charge $5 per square inch, provide 3 boards, have ENIG (gold) plating, and ship free anywhere in the world. If you want large boards and low cost or more than just a few boards, the Seeed Studio PCB https://www.seeedstudio.com/fusion_pcb.html will be your best alternative at US$4.90 for 10 boards up to 10*10cm